Actually, the concussion produced by the shot (shotgun, or any calibre rifle except maybe something of a very light load like a .22 'cap') hitting the water at a distance of a couple of feet above the water would most likely kill any and all fish in the barrel, no matter how fast they were swimming. If we were talking about a half inch or larger old fashioned calibre musket or pistol, game over... Of course if you are only thinking of a bow and arrow.... but I have always associated this expression with firearms.

Navy Mine Clearance Divers know that only James Bond can swim fast enough to get far enough away from the explosion to survive... Dropping standard hand grenades over the side of a warship will seriously upset any divers trying to lay mines under the keel... :-)

Keep it up McGrath, I'm on your side... :-) Always enjoy the intellectual stimulation. Obviously some of the GUEST contributors to this thread are w*****s...